SPECTRA GENERATOR FOR TEST AND CALIBRATION

Background

The present invention pertains to spectra and particularly infrared spectra of various substances. More particularly, the invention pertains to the generation of synthetic spectra and use of such spectra in testing and calibration of spectrometers.

The invention may be related to U.S. Patent No. 5,905,571, by Butler et al . issued May 18, 1999, and entitled "Optical Apparatus for Forming Correlation Spectrometers and Optical Processors"; U.S. Patent No. 5,757,536, by Ricco et al . , issued May 26, 1998, and entitled "Electrical-Programmable Diffraction Grating; and U.S. Patent No. 6,664,706, by Hung et al . , issued December 16, 2003, and entitled "Electrostatically- Controllable Diffraction Grating" ; which are herein incorporated by reference. The invention may also be related to U.S. Patent Application No. 10/352,828, by Hocker, filed January 28, 2003, and entitled

"Programmable Diffraction Grating Sensor"; and U.S. Patent Application No. 09/877,323, by Hocker et al . , filed June 8, 2001, and entitled "Apparatus and Method

for Processing Light", which are herein incorporated by reference .

Summary The invention may be an apparatus and method for the testing and calibration of spectrometers using generated synthetic spectra. These generated synthetic spectra may be used for other purposes such as scene generation.

Brief Description of the Drawing

Figure 1 is a schematic of a spectra generator having an electrically programmable reflective diffraction grating;

Figure 2 is an intensity versus wavelength graph of radiation from an example black body;

Figure 3 is an example spectrum that may represent the absorption of infrared light by a particular substance ;

Figure 4 is a diagram of an actual spectrum of HF, a simulated spectrum of HF and a displaced simulated spectrum of HF;

Figure 5 is a diagram of an actual spectrum of TCE, a simulated spectrum of TCE and a modified simulated spectrum of TCE; and

Figures 6a and 6b show end and top views, respectively, of an electrically adjustable grating.

Description

Spectrometers may be used to detect molecules in the atmosphere by observing the characteristic spectra of light absorbed by the molecules. Such spectrometers should be tested and calibrated with spectra that resemble the target molecules. Creating test spectra by using samples of the molecules, such particular species of them, may be inconvenient, time consuming and expensive. Additionally, using samples of the actual molecules may be hazardous if the species are toxic. Specifically, military systems used for standoff chemical agent detection need capabilities for test and calibration with actual spectral input representing the chemical agents to be detected, but without the need to use samples of actual toxic chemical agents.

In Figure 1, a generator 10 of spectra is shown. A light source 11 may output light 12 of a black body, that

is, broadband infrared light. A lens 13 may collimate light 12 for impingement on a diffractive grating 14. Electrically programmable diffractive grating 14 may reflect broadband light 12 as spectra light 15. The design may instead incorporate a transmissive grating in lieu of the reflective grating. The reflective grating may be generally referred to here.

Spectra light 15 may be detected by a spectrometer 16. Light 15 may be a synthesization of a light spectrum resulting from absorption by a specific substance. If spectrometer 16 is functioning properly, then it may identify that that spectrum light 15 to be that of the specific substance. The light 15 beam width may be adjusted with a beam expander or beam compressor so that light 15 is more effectively transmitted and detected by spectrometer 16.

The electrically programmable diffraction grating 14 may transform broadband light 12 into spectra light 15 in accordance with a dimension, such as the height of diffraction elements 17, relative to the base of diffractive gating 14. These dimensions of electrically programmable diffraction grating 14 in the Figures are not drawn to scale but are illustrative. The actual

number of elements 17 may be over 1000, e.g., 1024. Also, angle 18 may be a factor affecting the synthesized spectra 15. The spectra of light 15 generated may be a function of the heights of the elements 17 and of angle 18 of the direction of diffracted light 15 relative to the direction of incident light 12 impinging grating 14. Each element 17 may have a unique height .

The heights of the elements 17 may be adjusted in order to generate various spectra in diffracted light 15. The heights of the diffractive elements 17 may be set with electrical signals from a computer 19 via a connection 21 to an interconnection base 22 attached to grating 14. Computer 19 may be programmed to provide ready-made settings for the elements 17 to generate specific spectra of respective substances. Background about an electrically programmable diffraction grating may be disclosed in U.S. Patent Nos . 5,905,571 and 5,757,536, and U.S. Patent Application No. 09/877,323. For instance, if a request is input to computer 19 for a spectrum of CO, than an element pattern may be sent to grating 14 which may result in an adjustment of elements 17 so as to result in a spectrum of CO being in diffracted light 15 sent to a receptor 16 such as a

spectrometer. Elements 17 may be adjusted so as to reflect light 15 having spectra of more than one substance. Also, background may be added to the spectra of light 15. Spectrometer 16 may be tested with the reception of light 15 to determine detection capability of various substances among various backgrounds . Device 16 may be tested for identifying a spectrum of a particular substance or several substances buried in noise at one level or another. Computer 19 may provide spectra settings to elements 17 in a sequential fashion over a given period of time. Spectra for calibration of spectrometer 16 or other instrumentation may be provided via light 15. Further, a detection mechanism may be used added to device 16 to identify and verify the spectra being used for testing and calibrating spectrometers and the other instrumentation. Also, spectra may be generated for scene generation and the testing and/or calibration of microbolometers and other detection mechanisms . Figure 2 is a graph of intensity versus wavelength of infrared light. Curve 31 reveals a spectrum of black body source which may be light source 11 of generators 10 and 20. However, source 11 may emit other wavelengths of

the like, such in the ranges of visible or UV light. Figure 3 is an example of a spectrum 32 of light 15 or 24 after the light 12 is transmitted through a region containing a specific molecule. Light may be absorbed in spectral wavelengths in a pattern characteristic of that molecule .

Figure 4 shows a graph of three spectra of HF. The top spectrum may be regarded as an actual spectrum curve 51 of HF. Curve 52 may be a synthetic spectrum of HF as provided by spectra generator 10 or 20. Curve 53 is the same as curve 52 except that it is shifted to the right about 50 wave numbers. If the spectrum 52 is compared with the actual HF spectrum 51, and spectrum 52 is delayed periodically to the position of spectrum 53, spectrum 51 may be easier to detect when a comparison is done for calibrating the generator. Generators 10 and 20 may generate both a spectrum 52 and a displaced spectrum 53 of HF. This spectra displacement shifting may accommodate AC detection of an actual spectrum. Figure 5 shows an actual spectrum 61 of TCE and a synthetic spectrum 62 of TCE. Spectrum 62 may be provided by generators 10 and 20. Another provided spectrum 63 may effectively be spectrum 62 of TCE with

the 850 cm ^x absorption line removed by generator 10 or 20 due to an adjustment of the elements 17 in diffractive grating 14 or 23, respectively.

Figures 6a and 6b show aligned end and top views of the adjustable grating 14 that may be used in generator 10. A basic structure of this adjustable grating 14 may be like that of grating 23, except that grating 23 may have a transparent property rather than a reflective one. Elements 17 may be pulled down electrostatically by elements 71. One polarity of a voltage source may be connected to all of the elements 17 at support 73. The other polarity of the voltage source may be connected to an individual element 71 positioned relative to its corresponding element 17. Elements 17 may be like flexible tongs that have a natural resting position close structure 72 and a fixed structural connection to structure 73. A magnitude of a voltage applied across element 17 and 71 may determine the position of element 17 relative to element 71. The greater the voltage magnitude, element 17 may be drawn closer to element 71. Thus, all elements 17 may be individually adjusted to achieve a particular and unique diffractive grating 14 setting for providing a desired spectrum from generator

10. The voltage inputs to elements 71 may be individual and different from one another. The base 74 is insulated so that elements 71 may be electrically isolated from one another and connected to an external signal source such as computer 19. The various sets of voltage inputs with their respective combinations of magnitudes may be programmed in computer 19. The positions of elements 17 and consequently grating 14 may be dynamically changed in a manner to get the effect of going from spectrum 52 to spectrum 53 of Figure 5 or from spectrum 62 to spectrum 63 of Figure 6.

Although the invention has been described with respect to at least one illustrative embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications .